<i>In vitro</i> antimicrobial and antioxidant activities, essential oil composition, and <i>in silico</i> molecular modeling analysis of secondary metabolites from roots of <i>Verbascum sinaiticum</i>

Abstract

<i>Verbascum sinaiticum</i> is locally used to treat wound, stomachache, viral infection, cancer, sunstroke fever, abdominal colic, diarrhea, hemorrhage, anthrax, and hepatitis. The objective of this study was to identify the compounds and to evaluate the antimicrobial and antioxidant activity of the extracts and isolated compounds from <i>V. sinaiticum</i>. The ¹H-NMR, ¹³C-NMR, and DEPT-135 were used to elucidate the structures of isolated compounds. Essential oils were extracted by hydrodistillation method and their chemical analyses were performed by GC-MS. The broth microdilution method was used to evaluate the antimicrobial activity. The radical scavenging activity of the extracts and isolated compounds were evaluated using DPPH method. Silica gel column chromatographic separation of root extracts afforded seven known compounds: 3'-(4''-methoxy phenyl)-3'-oxo-propionyl hexadecanoate (<b>1</b>), harpagoside (<b>2</b>), pulverulentoside I (<b>3</b>), scrophuloside B4 (<b>4</b>), scropolioside A (<b>5</b>), scropolioside-D2 (<b>6</b>), and harpagide 6-<i>O</i>-β-glucoside (<b>7</b>), which are all reported from this species for the first time. The EO extracts from leaves and roots were the most susceptible to <i>Streptococcus agalactiae</i>, with a 2 mg/mL MIC. The EO from roots was effective against <i>Candida albicans</i> and <i>Trichophyton mentagrophytes</i>, with a MIC of 8 mg/mL. The MeOH and CH₂Cl₂/CH₃OH (1:1) root extracts showed the maximum activity against <i>S. epidermidis</i> with MIC values of 0.25 mg/mL. The strongest antibacterial effects were demonstrated against <i>Staphylococcus epidermidis</i>, which exhibited a 0.0625 mg/mL MIC for compound <b>1</b>. The strongest radical scavenging activity was exhibited by the methanol extract (IC₅₀ = 3.4 μg/mL), and compounds <b>4</b>, <b>6</b>, <b>5</b>, <b>3</b>, <b>7</b>, and <b>2</b> with IC₅₀ values of 3.2, 3.38, 3.6, 3.8, 4.2, and 4.7 μg/mL, respectively, in comparison with ascorbic acid (IC₅₀ = 1.3 μg/mL). The results of the molecular docking analysis of compounds revealed minimal binding energies range from -38.5 to -43.1 kJ/mol, -33.1 to -42.7 kJ/mol, -34.7 to -39.3.7 kJ/mol, -25.5 to -37.6 kJ/mol against human myeloperoxidase (PDB ID: 1DNU), murA enzyme (PDB ID: 1UAE), human topoisomerase II<i>β</i> (PDB ID: 4fm9), <i>S. epidermidis</i> FtsZ (PDB number: 4M8I) proteins, respectively. The docking results and the <i>in vitro</i> antibacterial activity are in good agreement. These findings show that the isolated compounds <b>2</b>-<b>7</b> can act as potential antioxidants and strong antibacterials against <i>Staphylococcus aureus</i> and <i>S. epidermidis</i>. As a result, <i>V. sinaiticum</i> root extracts have the potential to be effective in treating diseases caused by bacteria and free radicals, as long as further investigation has been suggested for the ultimate decision of this plant's potential candidate.